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  java备忘录之ArrayList源码分析
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  <p>ArrayList源码分析</p>
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<h2 id="ArrayList"><a href="#ArrayList" class="headerlink" title="ArrayList"></a>ArrayList</h2><p>ArrayList底层是用数组实现，能够自动扩展大小以适应存储元素的不断增加，它具有数组的一些特点，例如查找修改快而插入删除慢。</p>
<h3 id="成员变量"><a href="#成员变量" class="headerlink" title="成员变量"></a>成员变量</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//默认初始空间大小</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> DEFAULT_CAPACITY = <span class="number">10</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//空对象数组</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Object[] EMPTY_ELEMENTDATA = &#123;&#125;;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">// 和EMPTY_ELEMENTDATA一样，为了知道怎样扩充当第一个元素被添加</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = &#123;&#125;;</span><br><span class="line"></span><br><span class="line"><span class="comment">/*对象数组</span></span><br><span class="line"><span class="comment">基于数组实现，保存元素的数组使用 transient 修饰，该关键字声明数组默认不会被序列化。ArrayList </span></span><br><span class="line"><span class="comment">具有动态扩容特性，因此保存元素的数组不一定都会被使用，那么就没必要全部进行序列化。</span></span><br><span class="line"><span class="comment">ArrayList 重写了 writeObject() 和 readObject() 来控制只序列化数组中有元素填充那部分内容。*/</span></span><br><span class="line"><span class="keyword">transient</span> Object[] elementData; <span class="comment">// 非私有，以简化嵌套类访问</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//集合元素个数</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">int</span> size;</span><br></pre></td></tr></table></figure>

<h3 id="构造方法"><a href="#构造方法" class="headerlink" title="构造方法"></a>构造方法</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 传入初始容量的构造方法</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">ArrayList</span><span class="params">(<span class="keyword">int</span> initialCapacity)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (initialCapacity &gt; <span class="number">0</span>) &#123;</span><br><span class="line">        <span class="comment">//新建指定容量的Object类型数组</span></span><br><span class="line">        <span class="keyword">this</span>.elementData = <span class="keyword">new</span> Object[initialCapacity];</span><br><span class="line">    &#125; <span class="keyword">else</span> <span class="keyword">if</span> (initialCapacity == <span class="number">0</span>) &#123;</span><br><span class="line">        <span class="keyword">this</span>.elementData = EMPTY_ELEMENTDATA;</span><br><span class="line">    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> IllegalArgumentException(<span class="string">"Illegal Capacity: "</span>+</span><br><span class="line">                                           initialCapacity);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//不带参数的构造方法</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">ArrayList</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    <span class="comment">//将空的数组实例传给elementData</span></span><br><span class="line">    <span class="keyword">this</span>.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//传入外部集合的构造方法</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">ArrayList</span><span class="params">(Collection&lt;? extends E&gt; c)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//持有传入集合的内部数组的引用</span></span><br><span class="line">    elementData = c.toArray();</span><br><span class="line">    <span class="comment">//更新集合元素个数大小</span></span><br><span class="line">    <span class="keyword">if</span> ((size = elementData.length) != <span class="number">0</span>) &#123;</span><br><span class="line">        <span class="comment">// c.toArray might (incorrectly) not return Object[] (see 6260652)</span></span><br><span class="line">        <span class="comment">//判断引用的数组类型, 并将引用转换成Object数组引用</span></span><br><span class="line">        <span class="keyword">if</span> (elementData.getClass() != Object[]<span class="class">.<span class="keyword">class</span>)</span></span><br><span class="line"><span class="class">            <span class="title">elementData</span> </span>= Arrays.copyOf(elementData, size, Object[]<span class="class">.<span class="keyword">class</span>)</span>;</span><br><span class="line">    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">        <span class="comment">// 如果元素个数为0，引用指向空数组</span></span><br><span class="line">        <span class="keyword">this</span>.elementData = EMPTY_ELEMENTDATA;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="增删改查"><a href="#增删改查" class="headerlink" title="增删改查"></a>增删改查</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//增(添加)</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">add</span><span class="params">(E e)</span> </span>&#123;</span><br><span class="line">    <span class="comment">//添加前先检查是否需要拓展数组, 此时数组长度最小为size+1</span></span><br><span class="line">    ensureCapacityInternal(size + <span class="number">1</span>);  </span><br><span class="line">    <span class="comment">//将元素添加到数组末尾</span></span><br><span class="line">    elementData[size++] = e;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//增(插入)</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">add</span><span class="params">(<span class="keyword">int</span> index, E element)</span> </span>&#123;</span><br><span class="line">    <span class="comment">//插入位置范围检查</span></span><br><span class="line">    rangeCheckForAdd(index);</span><br><span class="line">    <span class="comment">//检查是否需要扩容</span></span><br><span class="line">    ensureCapacityInternal(size + <span class="number">1</span>);  </span><br><span class="line">    <span class="comment">//挪动插入位置后面的元素</span></span><br><span class="line">    System.arraycopy(elementData, index, elementData, index + <span class="number">1</span>,</span><br><span class="line">                     size - index);</span><br><span class="line">    <span class="comment">//在要插入的位置赋上新值</span></span><br><span class="line">    elementData[index] = element;</span><br><span class="line">    size++;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//删</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> E <span class="title">remove</span><span class="params">(<span class="keyword">int</span> index)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//index不能大于size</span></span><br><span class="line">    rangeCheck(index);</span><br><span class="line"></span><br><span class="line">    modCount++;</span><br><span class="line">    E oldValue = elementData(index);</span><br><span class="line"></span><br><span class="line">    <span class="keyword">int</span> numMoved = size - index - <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">if</span> (numMoved &gt; <span class="number">0</span>)</span><br><span class="line">        <span class="comment">//将index后面的元素向前挪动一位</span></span><br><span class="line">        System.arraycopy(elementData, index+<span class="number">1</span>, elementData, index,</span><br><span class="line">                         numMoved);</span><br><span class="line">    elementData[--size] = <span class="keyword">null</span>; <span class="comment">//置空引用 clear to let GC do its work</span></span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> oldValue;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">//查</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> E <span class="title">get</span><span class="params">(<span class="keyword">int</span> index)</span> </span>&#123;</span><br><span class="line">    rangeCheck(index);</span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> elementData(index);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//改</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> E <span class="title">set</span><span class="params">(<span class="keyword">int</span> index, E element)</span> </span>&#123;</span><br><span class="line">    rangeCheck(index);</span><br><span class="line"></span><br><span class="line">    E oldValue = elementData(index);</span><br><span class="line">    elementData[index] = element;</span><br><span class="line">    <span class="keyword">return</span> oldValue;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>每次添加一个元素到集合中都会先检查容量是否足够，否则就进行扩容，扩容的细节下面会讲到。我们先看具体增删改查要注意的地方。<br>增(添加)：仅是将这个元素添加到末尾。操作快速。<br>增(插入)：由于需要移动插入位置后面的元素，并且涉及数组的复制，所以操作较慢。<br>删：由于需要将删除位置后面的元素向前挪动，也会设计数组复制，所以操作较慢。<br>改：直接对指定位置元素进行修改，不涉及元素挪动和数组复制，操作快速。<br>查：直接返回指定下标的数组元素，操作快速。</p>
</blockquote>
<h2 id="扩容"><a href="#扩容" class="headerlink" title="扩容"></a>扩容</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">ensureCapacityInternal</span><span class="params">(<span class="keyword">int</span> minCapacity)</span> </span>&#123;</span><br><span class="line">    <span class="comment">//如果此时还是空数组</span></span><br><span class="line">    <span class="keyword">if</span> (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) &#123;</span><br><span class="line">        <span class="comment">//和默认容量比较, 取较大值</span></span><br><span class="line">        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">//数组已经初始化过就执行这一步</span></span><br><span class="line">    ensureExplicitCapacity(minCapacity);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">ensureExplicitCapacity</span><span class="params">(<span class="keyword">int</span> minCapacity)</span> </span>&#123;</span><br><span class="line">    modCount++;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//如果最小容量大于数组长度就扩增数组</span></span><br><span class="line">    <span class="keyword">if</span> (minCapacity - elementData.length &gt; <span class="number">0</span>)</span><br><span class="line">        grow(minCapacity);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * The maximum size of array to allocate.</span></span><br><span class="line"><span class="comment"> * 一些VM在数组中保留一些标题字。</span></span><br><span class="line"><span class="comment"> * 尝试分配更大的数组可能会导致</span></span><br><span class="line"><span class="comment"> * OutOfMemoryError: Requested array size exceeds VM limit</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="comment">//集合最大容量</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> MAX_ARRAY_SIZE = Integer.MAX_VALUE - <span class="number">8</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//增加数组长度</span></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">grow</span><span class="params">(<span class="keyword">int</span> minCapacity)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//获取数组原先的容量</span></span><br><span class="line">    <span class="keyword">int</span> oldCapacity = elementData.length;</span><br><span class="line">    <span class="comment">//新数组的容量, 在原来的基础上增加一半</span></span><br><span class="line">    <span class="keyword">int</span> newCapacity = oldCapacity + (oldCapacity &gt;&gt; <span class="number">1</span>);</span><br><span class="line">    <span class="comment">//检验新的容量是否小于最小容量</span></span><br><span class="line">    <span class="keyword">if</span> (newCapacity - minCapacity &lt; <span class="number">0</span>)</span><br><span class="line">        newCapacity = minCapacity;</span><br><span class="line">    <span class="comment">//检验新的容量是否超过最大数组容量</span></span><br><span class="line">    <span class="keyword">if</span> (newCapacity - MAX_ARRAY_SIZE &gt; <span class="number">0</span>)</span><br><span class="line">        newCapacity = hugeCapacity(minCapacity);</span><br><span class="line">    <span class="comment">// 拷贝原来的数组到新数组</span></span><br><span class="line">    elementData = Arrays.copyOf(elementData, newCapacity);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> <span class="title">hugeCapacity</span><span class="params">(<span class="keyword">int</span> minCapacity)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (minCapacity &lt; <span class="number">0</span>) <span class="comment">// overflow</span></span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> OutOfMemoryError();</span><br><span class="line">    <span class="keyword">return</span> (minCapacity &gt; MAX_ARRAY_SIZE) ?</span><br><span class="line">        Integer.MAX_VALUE :</span><br><span class="line">        MAX_ARRAY_SIZE;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>每次添加元素前会调用ensureCapacityInternal这个方法进行集合容量检查。在这个方法内部会检查当前集合的内部数组是否还是个空数组，如果是就新建默认大小为10的Object数组。如果不是则证明当前集合已经被初始化过，那么就调用ensureExplicitCapacity方法检查当前数组的容量是否满足这个最小所需容量，不满足的话就调用grow方法进行扩容。</p>
</blockquote>
<p>在grow方法内部可以看到，每次扩容都是增加原来数组长度的一半，扩容实际上是新建一个容量更大的数组，将原先数组的元素全部复制到新的数组上，然后再抛弃原先的数组转而使用新的数组。至此，我们对ArrayList中比较常用的方法做了分析，其中有些值得注意的要点：</p>
<p>ArrayList底层实现是基于数组的，因此对指定下标的查找和修改比较快，但是删除和插入操作比较慢。</p>
<p>构造ArrayList时尽量指定容量，减少扩容时带来的数组复制操作，如果不知道大小可以赋值为默认容量10。</p>
<p>每次添加元素之前会检查是否需要扩容，每次扩容都是增加原有容量的一半。</p>
<p>每次对下标的操作都会进行安全性检查，如果出现数组越界就立即抛出异常。</p>
<p>ArrayList的所有方法都没有进行同步，因此它不是线程安全的。</p>
<h2 id="Fail-Fast"><a href="#Fail-Fast" class="headerlink" title="Fail-Fast"></a>Fail-Fast</h2><p>modCount 用来记录 ArrayList 结构发生变化的次数。结构发生变化是指添加或者删除至少一个元素的所有操作，或者是调整内部数组的大小，仅仅只是设置元素的值不算结构发生变化。</p>
<p>在进行序列化或者迭代等操作时，需要比较操作前后 modCount 是否改变，如果改变了需要抛出 ConcurrentModificationException。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">writeObject</span><span class="params">(java.io.ObjectOutputStream s)</span></span></span><br><span class="line"><span class="function">    <span class="keyword">throws</span> java.io.IOException</span>&#123;</span><br><span class="line">    <span class="comment">// Write out element count, and any hidden stuff</span></span><br><span class="line">    <span class="keyword">int</span> expectedModCount = modCount;</span><br><span class="line">    s.defaultWriteObject();</span><br><span class="line"></span><br><span class="line">    <span class="comment">// Write out size as capacity for behavioural compatibility with clone()</span></span><br><span class="line">    s.writeInt(size);</span><br><span class="line"></span><br><span class="line">    <span class="comment">// Write out all elements in the proper order.</span></span><br><span class="line">    <span class="keyword">for</span> (<span class="keyword">int</span> i=<span class="number">0</span>; i&lt;size; i++) &#123;</span><br><span class="line">        s.writeObject(elementData[i]);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">if</span> (modCount != expectedModCount) &#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> ConcurrentModificationException();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="Vector"><a href="#Vector" class="headerlink" title="Vector"></a>Vector</h2><h3 id="同步"><a href="#同步" class="headerlink" title="同步"></a>同步</h3><p>它的实现与 ArrayList 类似，但是使用了 synchronized 进行同步。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">public synchronized boolean add(E e) &#123;</span><br><span class="line">    modCount++;</span><br><span class="line">    ensureCapacityHelper(elementCount + 1);</span><br><span class="line">    elementData[elementCount++] &#x3D; e;</span><br><span class="line">    return true;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">public synchronized E get(int index) &#123;</span><br><span class="line">    if (index &gt;&#x3D; elementCount)</span><br><span class="line">        throw new ArrayIndexOutOfBoundsException(index);</span><br><span class="line"></span><br><span class="line">    return elementData(index);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h3 id="ArrayList-与-Vector"><a href="#ArrayList-与-Vector" class="headerlink" title="ArrayList 与 Vector"></a>ArrayList 与 Vector</h3><p>Vector 是同步的，因此开销就比 ArrayList 要大，访问速度更慢。最好使用 ArrayList 而不是 Vector，因为同步操作完全可以由程序员自己来控制；<br>Vector 每次扩容请求其大小的 2 倍空间，而 ArrayList 是 1.5 倍。</p>
<h3 id="Vector-替代方案"><a href="#Vector-替代方案" class="headerlink" title="Vector 替代方案"></a>Vector 替代方案</h3><p>为了获得线程安全的 ArrayList，可以使用 Collections.synchronizedList(); 得到一个线程安全的 ArrayList。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">List&lt;String&gt; list &#x3D; new ArrayList&lt;&gt;();</span><br><span class="line">List&lt;String&gt; synList &#x3D; Collections.synchronizedList(list);</span><br></pre></td></tr></table></figure>
<p>也可以使用 concurrent 并发包下的 CopyOnWriteArrayList 类。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">List&lt;String&gt; list &#x3D; new CopyOnWriteArrayList&lt;&gt;();</span><br></pre></td></tr></table></figure>
<p>CopyOnWriteArrayList 是一种 CopyOnWrite 容器，从以下源码看出：读取元素是从原数组读取；添加元素是在复制的新数组上。读写分离，因而可以在并发条件下进行不加锁的读取，读取效率高，适用于读操作远大于写操作的场景。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">add</span><span class="params">(E e)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">final</span> ReentrantLock lock = <span class="keyword">this</span>.lock;</span><br><span class="line">    lock.lock();</span><br><span class="line">    <span class="keyword">try</span> &#123;</span><br><span class="line">        Object[] elements = getArray();</span><br><span class="line">        <span class="keyword">int</span> len = elements.length;</span><br><span class="line">        Object[] newElements = Arrays.copyOf(elements, len + <span class="number">1</span>); </span><br><span class="line">        newElements[len] = e;</span><br><span class="line">        setArray(newElements);</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125; <span class="keyword">finally</span> &#123;</span><br><span class="line">        lock.unlock();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">final</span> <span class="keyword">void</span> <span class="title">setArray</span><span class="params">(Object[] a)</span> </span>&#123;</span><br><span class="line">    array = a;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="meta">@SuppressWarnings</span>(<span class="string">"unchecked"</span>)</span><br><span class="line"><span class="function"><span class="keyword">private</span> E <span class="title">get</span><span class="params">(Object[] a, <span class="keyword">int</span> index)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> (E) a[index];</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

 
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                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.min.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>

<!-- MathJax -->

<script type="text/x-mathjax-config">
  MathJax.Hub.Config({
      tex2jax: {
          inlineMath: [ ['$','$'], ["\\(","\\)"]  ],
          processEscapes: true,
          skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code']
      }
  });

  MathJax.Hub.Queue(function() {
      var all = MathJax.Hub.getAllJax(), i;
      for(i=0; i < all.length; i += 1) {
          all[i].SourceElement().parentNode.className += ' has-jax';
      }
  });
</script>

<script src="https://cdn.jsdelivr.net/npm/mathjax@2.7.6/unpacked/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
<script>
  var ayerConfig = {
    mathjax: true
  }
</script>

<!-- Katex -->

<!-- busuanzi  -->


<script src="/js/busuanzi-2.3.pure.min.js"></script>


<!-- ClickLove -->

<!-- ClickBoom1 -->

<!-- ClickBoom2 -->

<!-- CodeCopy -->


<link rel="stylesheet" href="/css/clipboard.css">

<script src="https://cdn.jsdelivr.net/npm/clipboard@2/dist/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>


<!-- CanvasBackground -->


<script src="/js/dz.js"></script>



    
  </div>
</body>

</html>